Implement VS Code tree view checkbox API.

[tsmaeder]

What it does

Implements the ability to have check boxes on tree views in plugins

Fixes #12695

Contributed on behalf of STMicroelectronics

How to test

Install the attached VS Code extension. It contributes a a view called "Checkbox API Test". Open the view if necessary. Play around with checking/unchecking the checkboxes. The system automatically checks/unchecks parent and child checkboxes: see the VS Code doc linked in the issue for details.
The extension also contributes a preference "Manage Checkbox State Manually". When this preference is on, the automatic checking/unchecking is disabled. The extensions writes to the log, when it is notified of checkbox state changes. There is also a context menu action to check/uncheck the tree items programmatically.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[tsmaeder]

Here's the vsix:

checkboxapi-0.0.1.zip

and the source lives here:

https://github.com/tsmaeder/checkboxapi/

[msujew]

Looking quite good 👍

• The API is marked as supported in the comparator report
• The checkbox rendering works as expected
• Changing the preference sets the checkboxes into manual mode (requires reload of the plugin after changing the preference)

I have two minor comments below. Should be good to go afterwards 👍

[tsmaeder]

Thanks for the review.